1. Field of the Invention
This invention relates generally to tracking error detecting apparatus for an optical head and more particularly, is directed to a tracking error detecting apparatus for an optical head for use with an optical recording apparatus, an optical reproducing apparatus, an optical recording and/or reproducing apparatus.
2. Description of the Prior Art
FIG. 1 is a diagram showing an example of a prior art tracking error detecting apparatus for an optical head, in which reference letter OH generally designates an optical head on the whole. In FIG. 1, reference numeral 1 designates a semiconductor laser apparatus using a laser diode. In this semiconductor laser apparatus 1, a divergent laser beam L of ellipse-shape in cross section oscillated out or emitted from a laser light emission end face 1A thereof is made incident on a collimator lens 2 (which may be omitted as required) in which it is collimated as a parallel beam. This parallel beam is then made incident on a diffraction grating 3. From the diffraction grating 3 there are produced a zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1 (beams higher than + second-order or lower than - second-order are neglected). These beams are traveled through a non-polarizing beam splitter (half mirror) 4 (if a polarizing beam splitter is used, a 1/4 wavelength plate is provided between it and an objective lens 5) and is then made incident on the objective lens 5 thereby converged. The converged zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1 are made incident on a recording surface of an optical disc 6 used as an optical recording medium (including a magnetooptical recording medium) with a predetermined spacing (for example, 10 .mu.m) therebetween.
The zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1 reflected by the optical recording disc 6 are traveled through the objective lens 5 and introduced into the beam splitter 4, while a part of them is reflected on a reflecting surface 4a of the beam splitter 4 and thereby made incident on a photo-detector 7. The photo-detector 7 is formed of three photo-detecting sections to permit the zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1 to become incident thereon separately.
In the case of a tracking error detecting method known as a so-called three spots method, by calculating a difference between a pair of photo-detected outputs from a pair of photo-detecting sections on which the .+-. first-order beams L.sub.+1 and L.sub.-1 are incident, it is possible to obtain a tracking error signal which is corresponding to the tracking state of the zero-order beam L.sub.0 on the recording surface of the optical disc 6. From the photo-detecting section on which the zero-order beam L.sub.0 is made incident, there are produced a reproduced signal, a focusing error signal and so on.
Further, there is a method (see Japanese patent application No. 215860/1984) which is the improvement of a tracking error signal detecting method based on a so-called push-pull method. In this known method, of three beams, zero-order beam and one of the side beams provided at the both sides of the zero-order beam or all of three beams are used.
More specifically, this method uses such a photo-detector for the three beams the photo-detecting section of which is divided into equal two sections. Further, it is arranged such that when the beam spot by the zero-order beam lies on the track of the optical disc, the beam spots by the side beams provided at both sides of the zero-order beam lie on the lands or they are formed with a displacement of 1/2 track pitch. Accordingly, the outputs based on the difference between the detected outputs from the respective photo-detecting sections of the photo-detector relative to the beam spots, or the push-pull outputs become opposite in phase with respect to the beam spots formed by the zero-order beam and .+-. first-order beams. Whereas, the D.C. fluctuation components produced in the push-pull outputs by the lateral displacement of the objective lens and the skew of the disc become same in phase.
Accordingly, if a difference between the push-pull output PP.sub.0 from the photo-detector relative to the zero-order beam and the push-pull output PP.sub.1 or PP.sub.2 from the photo-detector for + first-order beam or - first-order beam is calculated, regardless of the lateral displacement of the objective lens and the skew of the optical disc, it is possible to obtain a tracking error signal which has no D.C. fluctuation components.
Using three push-pull outputs, PP.sub.0, PP.sub.1 and PP.sub.2, it is also possible to obtain the tracking error signal by caluculating PP.sub.0 -(G.sub.1 PP.sub.1 +G.sub.2 PP.sub.2). In this case, G.sub.1 and G.sub.2 represent constants which are presented by considering a difference between the gains of the photo-detectors.
Next, an example of the semiconductor laser apparatus 1 will be described with reference to FIG. 2.
Referring to FIG. 2, this semiconductor laser apparatus 1 is generally fixed to a header portion 8 made of metal such as copper and so on serving as one electrode which becomes a heat sink. In other words, in this example, the header portion 8 is formed of only the heat sink.
The structure of the laser chip of the semiconductor laser apparatus 1 will be described in the order of the upper layer to the lower layer. Reference numeral 1a designates an electrode layer, 1b an n-GaAs layer (substrate layer), 1c an n-Ga.sub.1-y Al.sub.y As layer (cladding layer), 1d a Ga.sub.1-x Al.sub.x As layer (active layer), le a p-Ga.sub.1-y Al.sub.y As layer (cladding layer) and 1f a p-GaAs layer. The above laser beam L is oscillated out from the active layer 1d. If the laser beam emission end face (wall surface) 1A of this semiconductor laser apparatus 1 is taken as a front, the width thereof is in a range from 100 to 300 .mu.m, the height (thickness) thereof is in a range from 80 to 100 .mu.m and the depth thereof is in a range from 200 to 300 .mu.m, respectively. The height of the active layer 1d from the upper surface of the header portion 8 is several .mu.m.
By the way, in practice, when the tracking error detecting method of not only the three spots method but also improved push-pull system is used, if in the optical disc, there is the skew in the tangential direction thereof, a D.C. fluctuation is produced in the tracking error signal so that it is not possible to detect the tracking error precisely.
After various researches, the present inventors et al. have studied the following causes.
Referring to FIG. 1, the zero-order beam L.sub.0 and the .+-. first-order beams L.sub.+1 and L.sub.-1 reflected on the optical disc 6 are traveled through the objective lens 5 and then reflected on the reflecting surface 4a of the beam splitter 4. Also they are traveled through the beam splitter 4 and introduced into the diffraction grating 3 which produces the corresponding zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1, separately. Then, they are traveled through the collimator lens 2 and are introduced into the semiconductor laser apparatus 1. The beam amount of the laser beam to be incident on the semiconductor laser apparatus 1 is large when the non-polarizing beam splitter is used and small when the polarizing beam splitter is used. In this case, in accordance with the position of the relative rotation angle between the laser beam emission end face 1A of the semiconductor laser apparatus 1 and the diffraction grating 3, there are cases that a central beam La and side beams Lb and Lc at the both sides of the central beam La to be incident on the semiconductor laser apparatus 1 are respectively arranged such that as shown in FIG. 3, the central beam La is positioned in the active layer 1d on the end face 1A and the both side beams Lb and Lc are positioned in the up and down direction on the straight line passing through the central beam La and perpendicular to the active layer 1d, that the central beam La and the both side beams Lb and Lc are all positioned in the horizontal direction positioned on the active layer 1b and that the straight line formed by connecting the central beam La and the both side beams Lb and Lc is positioned at a desired angle position which is intermediate between the above-described two cases. The central beam La and Lc are provided by diffracting again the zero-order beam L.sub.0 and .+-. first-order beams L.sub.+1 and L.sub.-1 by the diffraction grating 3 and superposing them in a mixed state.
When at least one of the both side beams Lb and Lc is made incident on the plane of the header portion 8, the plane of the header portion 8 is formed rough so that the beam is reflected irregularly. On the other hand, when at least one of the both side beams Lb and Lc is made incident on the laser beam emission end face 1A of the semiconductor laser apparatus 1, the end face 1A is excellent in reflectivity (for example, 10%) so that the beam is reflected on this end face 1A. As described above, the zero-order beam and the .+-. first-order beams incident on the semiconductor laser apparatus 1 are reflected thereon, diffracted again by the diffraction grating 3 and reached to the optical disc 6 so that on the photodetector 7, there is formed a complicated interference pattern.
The interference pattern is varied by a difference (phase difference) between the lengths of optical paths of the zero-order beam and the .+-. first-order beams. Thus, the interference pattern is varied by the change of the skew angle of the optical disc 6. Accordingly, the tracking error signal is varied by the change of the skew angle 60.degree. of the optical disc 6 so as to have a periodicity as, for example, shown in FIG. 4. In practice, as .vertline..alpha..vertline. is increased, the level of the tracking error signal Se is attenuated. When the both side beams Lb and Lc are both made incident on the laser beam emission end face 1A, the amplitude of the waveform corresponding to FIG. 4 becomes twice the amplitude shown in FIG. 4 and the phase thereof becomes different from the phase of FIG. 4.
Next, the analysis of the above-described interference pattern will be made with reference to FIG. 5 (from which the lens system is omitted).
Referring to FIG. 5, 1A shown by a solid line designates the laser beam emission end face which is, however, is inclined relative to the laser beam emission end face 1A positioned at the normal position shown by a broken line. Further, reference numeral 6 shown by a solid line designates an optical disc. This optical disc 6 has a skew and hence is inclined relative to the normal position shown by a broken line. The zero-order beam L.sub.0 is perpendicular to the laser beam emission end face 1A at the normal position and the recording surface of the optical disc 6 at the normal position. .theta. designates an angle of the + first-order beam L.sub.+1 relative to the zero-order beam L.sub.0. l.sub.1 designates an optical path length between the laser beam emission end face 1A and the diffraction grating 3. l.sub.2 designates an optical path length between the diffraction grating 3 and the recording surface of the optical disc 6. .DELTA.l.sub.1 and .DELTA.l.sub.2 respectively designate optical path length differences between the optical path length l.sub.1 and l.sub.2 of the zero-order beam L.sub.0 and + first-order beam L.sub.+1. .DELTA.l.sub.3 and .DELTA.l.sub.4 respectively designate an optical path difference caused by the skew of the optical disc 6 and an optical path difference caused by the skew of the laser beam emission end face 1A.
Further, g is taken as a phase difference between the zero-order beam L.sub.0 and + first-order beam L.sub.+1 in the diffraction grating 3. i.sub.0 and i.sub.1 are respectively taken as transmittances of the zero-order beam L.sub.0 and the + first-order beam L.sub.+1 in the diffraction grating 3. Reference letter t is taken as the transmittance of the half mirror 4 and r and f are respectively taken as reflectivities on the recording surface of the optical recording medium 6 and on the laser beam emission end face 1A.
Then, a complex amplitude of light at a point A on the recording surface of the optical disc 6 at which the + first-order beam L.sub.+1 is incident will be considered separately with respect to the following four cases.
(1) a.sub.1 : + first-order beam L.sub.+1 is made incident at the point A, directly. PA0 (2) a.sub.2 : zero-order beam which results from reflecting the zero-order beam L.sub.0 on the optical disc 6 and introducing it again into the diffraction grating 3 is reflected on the laser beam emission end face 1A and again introduced into the deffraction grating 3 to thereby produce the +first-order beam L.sub.+1. This + first-order beam L.sub.+1 is then incident at the point A. PA0 (3) a.sub.3 : + first-order beam is provided by reflecting the zero-order beam L.sub.0 on the optical disc 6 and introducing it again into the deffraction grating 3. This + first-order beam is reflected on the laser beam emission end face 1A and incident again on the diffraction grating 3 to thereby produce the zero-order beam. This zero-order beam is then made incident at the point A. PA0 (4) a.sub.4 : zero-order beam is provided by reflecting the + first-order beam L.sub.+1 on the optical disc 6 and introducing it again into the diffraction grating 3. This zero-order beam is reflected on the laser beam emission end face 1A and introduced again into the diffraction grating 3 to thereby produce the zero-order beam. Then, this zero-order beam is made incident at the point A.
The above-described a.sub.1 to a.sub.4 will be expressed by the following equations. EQU a.sub.1 =i.sub.1 t.multidot.exp {j(l.sub.1 +g+l.sub.2 +.DELTA.l.sub.2 +.DELTA.l.sub.3)} (1) EQU a.sub.2 =i.sub.0.sup.2 i.sub.1 t.sup.3 rf.multidot.exp [j{3(l.sub.1 +l.sub.2)+g+.DELTA.l.sub.2 +.DELTA.l.sub.3 }] (2) EQU a.sub.3 =i.sub.0.sup.2 i.sub.1 t.sup.3 rf.multidot.exp [j{3(l.sub.1 +l.sub.2)+g+2.DELTA.l.sub.1 +.DELTA.l.sub.2 +.DELTA.l.sub.3 +2.DELTA.l.sub.4 }] (3) EQU a.sub.4 =i.sub.0.sup.2 i.sub.1 t.sup.3 rf.multidot.exp [j{3(l.sub.1 +l.sub.2)+g+3(.DELTA.l.sub.2 +.DELTA.l.sub.3)+2.DELTA.l.sub.1 +2.DELTA.l.sub.4 }] (4)
In order to simplify the calculations, if the length by which the laser beam can be interferenced is selected to be lower than 2 (l.sub.1 +l.sub.2), the intensity I.sub.A of light at the point A is expressed by the following equation (5). ##EQU1## Further, when the both side beams Lb and Lc are both made incident of the laser beam emission end face 1A, if the + first-order beam L.sub.+1 is made incident at the point A on the recording surface of the optical disc 6 and the - first-order beam L.sub.-1 becomes incident on a point B which is symmetrical with respect to the zero-order beam L.sub.0, the intensity I.sub.A of the light at the point A is given as by Eq. (5), while an intensity I.sub.B of light at the point B is expressed by the following equation (6). ##EQU2##
As described above, the complex interference pattern is produced on the photo-detector 7. Particularly, when relative to the central beam La, the both sides beams Lb and Lc are arranged in the vertical direction and the side beam Lb becomes incident on the laser beam emission end face 1A and the side beam Lc becomes incident on the header portion 8, respectively, the side beam Lb is reflected on the laser beam emission end face 1A and the side beam Lc is reflected irregularly on the header portion 8 (whose surface is formed rough) so that with respect to the both side beams Lb and Lc which are returned to the semiconductor laser apparatus 1 and then incident on the side of the disc 6 again there is caused an unbalance, thus the D.C. fluctuation being produced in the tracking error signal. Similarly the D.C. fluctuation is produced in the tracking error signals of the three spots method and the push-pull method as mentioned before.